Polyhydroxyalkanoates (PHAs) are becoming more and more important, offering a promising alternative to conventional plastics, since they have favourable mechanical properties and, unlike other biopolymers, behave as thermoplastics. Furthermore, they can be recovered from renewable resources, such as biomass—namely either from biomass fermented by microorganisms producing PHAs during their life cycle as their food and energy reserves, or from biomass produced from or containing at least one crop-plant producing PHAs, such as genetically modified maize. Moreover, in the first case by selecting a strain of microorganisms and/or a carbon source for cultivation (saccharides/lipids), it is possible to obtain different compositions of PHAs, and as a result of providing suitable growth conditions for the employed microorganisms, the content of PHAs in their cells can reach up to 90%. In addition, when using the bacteria of the strain Cupriavidus necator H16 during the fermentation, it is possible to consume waste edible oils from thermal preparation of food as a carbon source, whose advantage is their low price and commercial availability. The best-known type of PHAs is polyhydroxybutyrate (PHB) and its copolymers containing 3-hydroxyvalerate and 3-hydroxyhexanoate.
Nowadays, there are known several methods of separating PHAs from biomass containing PHAs in which various solvents are used, such as partially halogenated hydrocarbons (see e.g. EP 0015123 and U.S. Pat. No. 4,324,907), carbonates (see e.g. U.S. Pat. No. 4,101,533 and U.S. Pat. No. 4,140,741), higher alcohols and their esters (see e.g. US 2007/0161096, WO 97/07229 and WO 2009/114464) and other substances, such as esters of dicarboxylic and tricarboxylic acids and gamma-butyrolactone (see e.g. U.S. Pat. No. 4,968,611), etc., which extract PHAs from biomass and from which PHAs are subsequently separated in a suitable method. The disadvantage of these processes is the fact that due to the character of the solvents employed they take place at higher temperatures which at the same time cause thermal degradation of the isolated PHA.
From this point of view, the most advantageous solution is using extraction agents based on chlorinated hydrocarbons, since that enables to separate PHA at low temperatures (generally ranging approximately from 100 to 120° C.), at which thermal degradation of PHA does not occur yet (see e.g. U.S. Pat. No. 4,310,684, EP 0014490, U.S. Pat. No. 4,562,245, U.S. Pat. No. 4,705,604 and U.S. Pat. No. 5,213,976). However, during testing these methods, it was found that extraction agents based on chlorinated hydrocarbons extract, apart from PHAs, also other components from the biomass, which during subsequent separation precipitate in water together with PHAs, thus substantially decreasing their final purity. Consequently, the purity reaches approximately 90% at the most (see e.g. the comparative example 1 hereinafter). In addition, in the method according to U.S. Pat. No. 5,213,976, insufficient water turbulence during precipitation leads to the formation of large particles of PHA, which have to be additionally disintegrated.
An alternative method in which the contamination of PHAs with undesired components of the biomass is eliminated is precipitation of PHAs with an organic solvent. However, costs of further disposal of this organic solvent (which is used in considerable excess) are high, and PHAs precipitate in the form of gel having a high moisture content, and so they have to be further dried.